Alpha 1 in the Ubuntu 16.04 'Xenial Xerus' development cycle is now available to download (should you feel so inclined).
The first alpha releases in the Ubuntu 15.04 development cycle are now available to download for testing.
The first alpha release in the Ubuntu 12.04 LTS release cycle is now available for download. Installing the Alpha release is not recommended due to its early yet rapid developmental stage. Ubuntu 12.04 LTS is scheduled for release in April 26th 2012.
The first alpha release of the Ubuntu 11.10 development cycle is now available for developers, testers and bug-hunters to download.